Heroku CLJ Docker

Using Docker builds with heroku.yml beta feature to deploy Dockerized Clojure app to Heroku.

Motivation

Heroku has great support for Clojure using lein build tool but not all projects want to be tied to lein or they require more flexibility with build and deployment than Heroku standard tooling for Clojure provides. This project is an example how to use Docker with deps and clj to build and run Clojure applications in Heroku. Same strategy can be used with any other build tool.

Usage

Prerequisites

• docker
• docker-compose
• Heroku account
• heroku-cli

Setup

heroku update beta
heroku plugins:install @heroku-cli/plugin-manifest
heroku login

git clone https://github.com/vharmain/heroku-clj-docker.git
cd heroku-clj-docker

heroku create insert-app-name-here --manifest
heroku git:remote -a heroku-clj-docker

Deploy to Heroku

Commiting any change to heroku remote triggers deploy.

git add -A
git commit -m "My first commit"
git push heroku master

# Watch logs
heroku logs --tail

Run locally

docker-compose up web

Observations and notes

• One of the least painful Docker deployment experiences I've had. Heroku provides excellent ergonomics what comes to developer experience. Commands such as heroku logs --tail and heroku run bash work similarly to 'normal' buildpacks.
• I first tried running the app directly with clj (no uberjaring). During build time .m2 folder resolves under $HOME/.m2 but on container start clj/maven tries to lookup .m2 relative to WORKDIR defined in Dockerfile. In practice this meant that all dependencies would be downloaded on each restart. Workaround to this is to either build uberjar on build time or place the app directly to users home folder and use it as the WORKDIR (see Dockerfile-no-uberjar) so Maven can find .m2. What is causing this? I'm not sure but I'm suspecting that some ENV var supplied by Heroku is getting Maven confused where $HOME is at runtime. I wasn't able to reproduce this locally with docker-compose.
• Heroku has some limitations with Docker.